Accuracy of Weather Forecasts in Time

Short-range weather forecasts tend to be more accurate than long-range weather forecasts.

timeanddate.com explains why forecasts for today’s weather are generally more accurate than weather forecasts further ahead in the future. The different types of forecast ranges are also briefly explained.

Forecast Range Types

Before examining why longer-term forecasts are less accurate than current forecasts, timeanddate.com looks at basic definitions on the types of weather forecasts made in relation to time:

A short-range forecast is a weather forecast made for a time period up to 48 hours.

Extended forecasts are for a period extending beyond three or more days (eg. a three to five-day period) from the day of issuance.

Medium range forecasts are for a period extending from about three days to seven days in advance.

Long-range forecasts are for a period greater than seven days in advance but there are no absolute limits to the period.

Short-range forecast predictions, where the forecast is made for a time period for today and/or tomorrow (up to 48 hours), are generally more accurate than the other types of forecasts.

Why Are Longer-Term Forecasts Less Accurate?

Weather forecasts still have their limitations despite the use of modern technology and improved techniques to predict the weather. For example, weather forecasts for today or tomorrow are likely to be more dependable than predictions about the weather about two weeks from now. Some sources state that weather forecast accuracy falls significantly beyond about 10 days.

Weather forecasting is complex and not always accurate, especially for days further in the future, because the weather can be chaotic and unpredictable. For example, rain or snow cannot always be predicted with a simple yes or no. Moreover, the Earth’s atmosphere is a complicated system that is affected by many factors and can react in different ways.

If weather patterns are relatively stable, the persistence method of forecasting provides a relatively useful technique to predict the weather for the next day. If it is hot and sunny on one day, it is likely to be hot and sunny the next. However, the weather in many parts of the world is more unpredictable and changeable than that, particularly in the mid-latitudes where depressions influence much of the weather.

Depressions, sometimes called mid-latitude cyclones, are areas of low pressure located between 30 degrees and 60 degrees latitude. Depressions develop when warm air from the sub-tropics meets cold air from the Earth’s polar regions. Depressions usually have well defined warm and cold fronts, as the warm air is forced to rise above the cold air.

Improving Predictions

Weather observation techniques have improved and there have been technological advancements in predicting the weather in recent times. On average, a five-day weather forecast of today is as reliable as a two-day weather forecast 20 years ago. Despite this major scientific and technical progress, many challenges remain regarding long-term weather predictability.

The accuracy of individual weather forecasts varies significantly. The challenges include finding out more about individual forecasts’ changing uncertainties as well as improving forecasting skills in areas where progress has been difficult (such as heavy rainfall and the genesis, intensity and structure of tropical cyclones).